In the last decade, our understanding of the processes underlying plant response to drought, at the molecular and whole-plant levels, has rapidly progressed. Here, we review that progress. We draw attention to the perception and signalling processes (chemical and hydraulic) of water deficits. Knowledge of these processes is essential for a holistic understanding of plant resistance to stress, which is needed to improve crop management and breeding techniques. Hundreds of genes that are induced under drought have been identified. A range of tools, from gene expression patterns to the use of transgenic plants, is being used to study the specific function of these genes and their role in plant acclimation or adaptation to water deficit. However, because plant responses to stress are complex, the functions of many of the genes are still unknown. Many of the traits that explain plant adaptation to drought - such as phenology, root size and depth, hydraulic conductivity and the storage of reserves - are those associated with plant development and structure, and are constitutive rather than stress induced. But a large part of plant resistance to drought is the ability to get rid of excess radiation, a concomitant stress under natural conditions. The nature of the mechanisms responsible for leaf photoprotection, especially those related to thermal dissipation, and oxidative stress are being actively researched. The new tools that operate at molecular, plant and ecosystem levels are revolutionising our understanding of plant response to drought, and our ability to monitor it. Techniques such as genome-wide tools, proteomics, stable isotopes and thermal or fluorescence imaging may allow the genotype-phenotype gap to be bridged, which is essential for faster progress in stress biology research.

Understanding plant responses to drought — from genes to the whole plant

1. Light and Energy. 2. Organization and Structure of Photosynthetic Systems. 3. History and Development of Photosynthesis. 4. Photosynthetic Pigments-Structure and Spectroscopy. 5. Antenna Complexes and Energy Transfer Processes. 6. Reaction Center Complexes. 7. Electron Transfer Pathways and Components. 8. Chemiosmotic Coupling and ATP Synthesis. 9. Carbon Metabolism. 10. Genetics, Assembly and Regulation of Photosynthetic. Systems. 11. Origin and Evolution of Photosynthesis. Appendix 1. Light, Energy and Kinetics

Molecular mechanisms of photosynthesis

Two major energy-related problems confront the world in the
next 50 years. First, increased worldwide competition for
gradually depleting fossil fuel reserves (derived from past
photosynthesis) will lead to higher costs, both monetarily and politically. Second, atmospheric CO_2 levels are at their highest recorded level since records began. Further increases are predicted to produce large and uncontrollable impacts on the world climate. These projected impacts extend beyond climate to ocean acidification, because the ocean is a major sink for atmospheric CO2.1 Providing a future energy supply that is secure and CO_2-neutral will require switching to nonfossil energy sources such as wind, solar, nuclear, and geothermal energy and developing methods for transforming the energy produced by these new sources into forms that can be stored, transported, and used upon demand.

Frontiers, Opportunities, and Challenges in Biochemical and Chemical Catalysis of CO2 Fixation

Increasing the yield potential of the major food grain crops has contributed very significantly to a rising food supply over the past 50 years, which has until recently more than kept pace with rising global demand. Whereas improved photosynthetic efficiency has played only a minor role in the remarkable increases in productivity achieved in the last half century, further increases in yield potential will rely in large part on improved photosynthesis. Here we examine inefficiencies in photosynthetic energy transduction in crops from light interception to carbohydrate synthesis, and how classical breeding, systems biology, and synthetic biology are providing new opportunities to develop more productive germplasm. Near-term opportunities include improving the display of leaves in crop canopies to avoid light saturation of individual leaves and further investigation of a photorespiratory bypass that has already improved the productivity of model species. Longer-term opportunities include engineering into plants carboxylases that are better adapted to current and forthcoming CO2 concentrations, and the use of modeling to guide molecular optimization of resource investment among the components of the photosynthetic apparatus, to maximize carbon gain without increasing crop inputs. Collectively, these changes have the potential to more than double the yield potential of our major crops.

/pdf/improving-photosynthetic-efficiency-for-greater-yield-4camhk54us.pdf

Improving Photosynthetic Efficiency for Greater Yield

The yield potential ( Y p ) of a grain crop is the seed mass per unit ground area obtained under optimum growing conditions without weeds, pests and diseases. It is determined by the product of the available light energy and by the genetically determined properties: efficiency of light capture ( e i ), the efficiency of conversion of the intercepted light into biomass ( e c ) and the proportion of biomass partitioned into grain ( h ). Plant breeding brings h and e i close to their theoretical maxima, leaving e c , primarily determined by photosynthesis, as the only remaining major prospect for improving Y p . Leaf photosynthetic rate, however, is poorly correlated with yield when different genotypes of a crop species are compared. This led to the viewpoint that improvement of leaf photosynthesis has little value for improving Y p . By contrast, the many recent experiments that compare the growth of a genotype in current and future projected elevated [CO 2 ] environments show that increase in leaf photosynthesis is closely associated with similar increases in yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing e c by improving photosynthetic efficiency were explored, ranging from altered canopy architecture to improved regeneration of the acceptor molecule for CO 2 . Collectively, these changes could improve e c and, therefore, Y p by c . 50%. Because some changes could be achieved by transgenic technology, the time of the development of commercial cultivars could be considerably less than by conventional breeding and potentially, within 10‐15 years.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2005.01493.x

Can improvement in photosynthesis increase crop yields

C4 photosynthesis: a unique elend of modified biochemistry, anatomy and ultrastructure

In C4 plants carbonic anhydrase catalyzes the critical first step of C4 photosynthesis, the hydration of CO2 to bicarbonate. The maximum activity of this enzyme in C4 leaf extracts, measured by H+ production with saturating CO2 and extrapolated to 25°C, was found to be 3,000 to 10,000 times the maximum photosynthesis rate for these leaves. Similar activities were found in C3 leaf extracts. However, the calculated effective activity of this enzyme at in vivo CO2 concentrations was apparently just sufficient to prevent the rate of conversion of CO2 to HCO3− from limiting C4 photosynthesis. This conclusion was supported by the mass spectrometric determination of leaf carbonic anhydrase activities.

/pdf/carbonic-anhydrase-activity-in-leaves-and-its-role-in-the-jmsarljcy9.pdf

Carbonic Anhydrase Activity in Leaves and Its Role in the First Step of C4 Photosynthesis

We sought to characterize the inorganic carbon pool (CO(2) plus HCO(3) (-)) formed in the leaves of C(4) plants when C(4) acids derived from CO(2) assimilation in mesophyll cells are decarboxylated in bundle sheath cells The size and kinetics of labeling of this pool was determined in six species representative of the three metabolic subgroups of C(4) plants The kinetics of labeling of the inorganic carbon pool of leaves photosynthesizing under steady state conditions in (14)CO(2) closely paralleled those for the C-4 carboxyl of C(4) acids for all species tested The inorganic carbon pool size, determined from its (14)C content at radioactivity saturation, ranged between 15 and 97 nanomoles per milligram of leaf chlorophyll, giving estimated concentrations in bundle sheath cells of between 160 and 990 micromolar The size of the pool decreased, together with photosynthesis, as light was reduced from 900 to 95 microeinsteins per square meter per second or as external CO(2) was reduced from 400 to 98 microliters per liter A model is developed which suggests that the inorganic carbon pool existing in the bundle sheath cells of C(4) plants during steady state photosynthesis will comprise largely of CO(2); that is, CO(2) will only partially equlibrate with bicarbonate This predominance of CO(2) is believed to be vital for the proper functioning of the C(4) pathway

/pdf/mechanism-of-c4-photosynthesis-the-size-and-composition-of-2s74e1iomi.pdf

Mechanism of C4 Photosynthesis: The Size and Composition of the Inorganic Carbon Pool in Bundle Sheath Cells

Activity, location, and role of asparate aminotransferase and alanine aminotransferase isoenzymes in leaves with C4 pathway photosynthesis.

A theoretical model of the composition of the inorganic carbon pool generated in C4 leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO2 and O2 in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO2 concentration of 70 micromolar during steady state photosynthesis in a typical C4 plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO2. Under these circumstances the flux of carbon through the C4 acid cycle would have to exceed the net rate of CO2 assimilation by 15.5%. With the calculated O2 concentration of 0.44 millimolar, the potential photorespiratory CO2 loss in bundle sheath cells would be about 3% of CO2 assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO3−, the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO2 and O2 diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C4 plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C4 acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate.

/pdf/mechanism-of-c4-photosynthesis-a-model-describing-the-5a9txio7cv.pdf

Marshall D. Hatch

Papers

C4 photosynthesis: a unique elend of modified biochemistry, anatomy and ultrastructure

Carbonic Anhydrase Activity in Leaves and Its Role in the First Step of C4 Photosynthesis

Mechanism of C4 Photosynthesis: The Size and Composition of the Inorganic Carbon Pool in Bundle Sheath Cells

Activity, location, and role of asparate aminotransferase and alanine aminotransferase isoenzymes in leaves with C4 pathway photosynthesis.

Mechanism of C4 Photosynthesis: A Model Describing the Inorganic Carbon Pool in Bundle Sheath Cells